Captain America would probably be very reflective. He might see his past self as a reminder of his long journey and how much he has changed. He could try to offer some words of wisdom to his younger self.
Some gods might be entertained. For instance, Hermes, being the trickster god, might enjoy seeing the different and often humorous takes on the gods in the fanfiction.
Issei could be a bit nervous at first. He might worry about what his classmates will think. But if they show positive reactions, he could become more confident and start sharing more about his writing. However, if they react negatively, he may feel a bit down and might even regret that they saw it.